The direct arc electric furnace is the type most commonly used today in the production of low sulfur, low inclusion iron which is particularly useful in the casting industry. It is primarily a scrap melting furnace made possible by the use of extremely high amperage exceeding 100,000 amps. Cylindrical, solid graphite electrodes are suspended above the furnace shell and extend down through ports in the furnace roof and exhaust hood. The electrodes are used to conduct the electric current which passes from one electrode through an arc to the metal charge, through the metal charge, and thence out through an arc to another electrode.
The electrodes are positioned at various distances from the solid or molten metal to be heated, depending on the stage of heating during an electric arc furnace run. For example, during the initial cold melting stage of solid scrap metal, the electrodes are typically brought closer than three inches to the charge and may be contacting some of the scrap. The power and current passing through the electrodes is at least twice that which is used during the hot refining stage of the entire heating sequence. The electrodes essentially burn a hole through the scrap with electrodes being lowered closer to the bottom of the furnace where a molten pool is forming. Continued melting takes place by solid metal falling into the holes as they widen, adding to the pool.
This tremendous surge of power and close proximity of the electrodes to the metal charge creates a violent electrical discharge in this early stage of the heating sequence. Considerable generation of gas takes place, along with a large amount of metal dust. The furnace gas that is emitted is characteristically dry, with a very high electrical resistivity. Therefore, with the large amount of fume generation and dust, a cleansing station is necessary to process the fumes and gather the dust before releasing the gases to the atmosphere.
Various means have been used for collection, including hoods mounted directly on the furnace, hoods mounted separately over the top of the furnace, offtake pipes that apply suction to the furnace directly through an opening in the roof or sidewall of the furnace, a variation of the direct offtake called a "snorkle" that collects fumes by stack effect through an opening in the furnace roof, and even evacuation of the entire building in which the furnace is housed.
The use of the roof as a hood over the top of the furnace appears to offer the best functional advantage, from the standpoint of efficiency of collection, and minimal effects on metallurgical conditions. Such a roof evacuation system delivers the collected gases to a bag house consisting of a plenum and a number of filtering bags or compartments. Since the exhaust gas is laden with particles widely varying in size from sibmicron mean particle size to very large particle sizes, the cloth type filter or bag appears to be the most efficient, economical means of cleansing the gas. It has become typical for such bags to be constructed of polyester synthetic woven cloths for the filter medium. However, such filtering bags have a temperature limitation of typically 300.degree. F. If such temperature is exceeded, the effectiveness of the bags may be destroyed by a weakening of the cloth and eventual holes.
To protect the bag filters from temperatures in excess of about 300.degree. F., it is typical to employ a thermal sensing device to open a bypass damper or gate upon sensing the excess temperature of 300.degree. F. or more. When the treatment station is bypassed, the fumes from the electric arc furnace will be either sent to the atmosphere or to other treatment or holding zones.
The bypass, being independent of flow, often exaggerates the volume of the bypassed gases. This may result from high energy particles in the form of sparking slag particles which bombard the thermocouple. These momentary thermal pulses can occur quite frequently in a given time span, much quicker than the response cycle of the bypass system employing a mechanical actuator. As a result, the bypass system tends to stay open for a considerable period of time which greatly reduces the effectiveness of the filtering system, particularly during the cold melting stage of the furnace run.